1. Field of the Invention
The instant invention relates to methods of and apparatus for producing flat, biaxially oriented, polymer sheet. More particularly, the instant invention relates to methods of and apparatus for producing flat, biaxially oriented, polymer sheet, wherein the sheet has substantially uniform strain distribution through the thickness of the sheet.
2. Technical Considerations and Prior Art
Polypropylene polymers, such as those sold under the trademark BEXOR, have numerous advantages over unoriented polymers in that they have higher strength, improved stiffness, increased toughness and easier thermoformability. However, for several reasons it has proved relatively difficult to produce sheets of polypropylene materials by solid state extrusion processes which retain this flatness when heated for forming. This is because orientation of polymers through the thickness of sheets produced by solid state extrusion is not uniform causing the sheets to curve and buckle, resulting in difficulties in handling the sheets when preheated for forming. During solid state extrusion, it is necessary to apply compensation pressure to balance extrusion pressures. This requires complex, expensive machinery. Moreover, since the polymer material cannot be quenched soon after extrusion, it is very difficult to biaxially orient amorphous polymers in compression.
U.S. Pat. No. 4,282,277, assigned to the assignee of the instant invention, discloses using a mandrel in combination with a conical die to produce a tubular product. The tubular product is then cut, flattened and annealed utilizing expensive flattening and annealing equipment. While the mandrel--conical die approach results in relatively good biaxial orientation of structures, it is necessary to expend considerable effort to remove the curvature of the resulting product. Even when the curvature has been removed by reconfiguring the extruded tube into a flat sheet, there are residual stresses and strains which tend to subsequently assert themselves, resulting in latent difficulties when using the product.
Attempts have been made to extrude high density polyethylene while in the solid state utilizing dies with flat inner surfaces which converge in a first transverse direction and diverge in a second transverse direction normal to the first transverse direction. However, it has been found that the material extrudes non-uniformly due to friction between the specimen and the die which prevents the top and bottom surfaces of the material from extending to the two diverging sides of the die. In specimens produced with flat dies, the sides of the surfaces bulge due to surface friction. With the exception of the center of the extrudate, there is considerable distortion in the extrudate resulting in non-uniform sheet material which cannot be used for most applications.